Two former Parramatta Eels players are accused of harbouring semi-automatic weapons and possessing more than half-a-million dollars in cash after dramatic arrests in Sydney's Centennial Park yesterday.

Despite rain, protesters gathered around the Washington Monument for a festive day of music, speeches and teach-ins by scientists disturbed by the rise of so-called "alternative facts" around crucial issues like climate change following Mr Trump's election.

"We have no Planet B," read one of the signs.

A march was held in Boston. (AFP)

Many demonstrators sported "Keep Our Science Great" caps as they arrived from around the country on Earth Day to highlight the importance of science to daily lives.

Protesters marched to the US Capitol to carry their pro-science message.

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"Science is political but it is not partisan. Science serves all of us. Together we can - dare I say it - save the world! Let's march!" declared television personality Bill Nye the Science Guy, who currently heads the Planetary Society.

The movement was echoed in hundreds of events across the United States and around the world, from Sydney to Accra.

At a time when the Earth has marked three consecutive years of record-breaking heat, and ice is melting at an unprecedented rate at the poles, risking massive sea level rise in the decades ahead, some marchers said it was more important than ever for scientists to communicate and work toward solutions to curb fossil fuel emissions.

"Ditch the jargon," advised Tyler DeWitt, the star of a popular YouTube show on science.

Protesters in Sydney. (AAP)

"Make it understandable. Make people care. Talk to them, not at them. We cannot complain about slashed funding if we can't tell taxpayers why science matters."

Organisers stressed the protest was non-partisan. But concerns about the challenges to the role of science in society have spiked under Mr Trump's presidency.

He has proposed deep cuts in funding for scientific research, elevated opponents of climate pacts and environmental regulations to cabinet-level positions, and drawn support from conservative Christians who challenge the teaching of evolution in US schools.

"If this president has his way, science is in danger but I think there will be a lot of resistance from Congress," said Elisabeth Johnston, a retired biologist.

Mr Trump issued a statement defending his administration's policies as aimed at protecting the environment "without harming America's working families."

"I am committed to keeping our air and water clean but always remember that economic growth enhances environmental protection. Jobs matter!" the Republican president said in a tweet, without acknowledging the massive crowds of marchers.

In London, hundreds of people marched from the Science Museum to the Houses of Parliament, holding signs with messages including "Science is Sexy" and "Less Invasions, More Equations".

The London rally was attended by actor Peter Capaldi, who plays TV's time-travelling hero of science, Dr Who.

In Ghana, organizers used the day for a teach-in at a beachside hotel in Accra about environmental issues of local concern such as the impact of plastic waste on the environment.

"It's killing our fish, we have flooding in our communities, we have a rise of environmental diseases," said Cordie Aziz, an American activist involved in plastics recycling.

Vocal protesters in Sydney wearing white lab coats called on politicians to support the scientific community.

"We need thinkers not deniers," read one banner.

Demonstrators turned out across Australia, in Melbourne, Brisbane, Perth, Adelaide and other cities, as well as Wellington and Auckland in New Zealand.

"In this day and age, there's so much fake news and alternate facts going around that it's important to remember that science is what has built the society we know today," Parissa Zand, who was at the Sydney march with her molecular biologist mother, told AFP.

Protestors in major university cities in Europe posted pictures on Twitter of marches in Bonn, Helsinki, Munich and Stockholm.

In Paris, a banner in French read: "We are the resistance against the orange menace in Washington! Defend science!"

Other rallies were scheduled in Brazil, Canada, Japan, Mexico, Nepal, Nigeria and South Korea.

"Seeing the assault on fact-based thinking, scientists are energized," Paul Hanle, chief executive officer of Climate Central, an independent organisation of scientists and journalists, wrote in an op-ed this week.

Scientists "are not famous for their camaraderie," said professor of carbon management at the University of Edinburgh, David Reay. "We are trained to question, criticize and, where needed, contest each other's work.

"That we are now marching together is testament to just how threatened our disparate community feels."